This invention relates to the field of a variable laser light attenuators for use in adjustable typographic composing and printing machines.
In order to illuminate a recording medium with a desired intensity in the field of typographical composition or printing and thus to achieve a desired blackness, e.g., in the case of a recording medium containing silver halides and thereby generally ensure sharp resolution of the illuminated indicia, it is desirable to attenuate the beam emitted by a light source. This requirement is of even greater importance, in particular if the exposure time of the medium to the light is variable. In order to attenuate the beam illuminating the recording medium when necessary, the prior art generally teaches employing a filter wheel for instance, one which is equipped with filters of different degrees of absorption adapted to be selectively rotated for a desired absorption level into the path of the beam. This, however, only permits rough adjustment should the number of filters available be unduly limited.
When using a laser light source, continuous attenuation of the beam illuminating the recording carrier proves to be somewhat more difficult than when flash lights or cathode ray tubes are used since the brightness of the latter can be varied by varying electric parameters.
Various means are known for attenuating laser light continuously, and if possible, without any other changes in the laser light beam. Such attenuators typically include, in particular, infrared polarizers comprising two trapezoidal prisms into which a body of CaCO.sub.3 is cut at what is known as Brewster's Angle, i.e., the polarizing angle. The two prisms are mounted in a predetermined, fixed, mutual position by a holder. The holder is adapted to be moved in its entirety in the beam path of the laser to produce a variable attenuation. This polar, like other known attenuators, is designed to not be dependent, if at all possible, on the polarization of the light to be attenuated. The properties of the substance contained in this known polarizer were exploited for this purpose. However, the costs of manufacturing the optical parts of the attenuator are excessively costly since the attenuator must be produced from selected, naturally grown crystal.
Another known polarizer for effecting the variable attenuation of infrared light consists of six to ten plates disposed at Brewster's Angle and inclined such that the beam deflection caused by half of the plates is restored by the other half. The attenuation is due to the fact that, at the Brewster Angle, the light in the p-polarization is transmitted almost completely, whereas light in the s-polarization is reflected on each surface of the plates. The attenuation resulting due to this reflection is in accordance with Fresnel's Law. The comparatively large number of optical surfaces is disadvantageous in this technology, as considerable adjustment work is required, among other things, and also because there is a tendency for the light beam to undergo comparatively high deflection (beam steering).
To avoid further disadvantages of the aforementioned stack-of-plates polarizers constructed of plane plates, the prior art teaches to provide an attenuator with four wedge-shaped plates which are mutually inclined in pairs. To adjust the attenuation, the mutual inclination of the two plate pairs is changed; i.e., the plate pairs are rotated in opposite directions. This requires a relatively high mechanical cost outlay if other disadvantages (beam deflection) are to be avoided.
A feature all known laser light attenuators have in common is that the resultant outlay for use in commercial typographic composing and printing machines is too high.